A frequency converter is an electric device with which it is possible to feed a load with a voltage that has a variable frequency. Frequency converters are typically used with motors to control them at an altering frequency or when transferring electric power back to the network, in which case the converter generates a voltage whose frequency corresponds to that of the network.
Today, IGB transistors (Insulated Gate Bipolar Transistor, IGBT), which are fast, gate-controlled components, are typically used as power semiconductors of converters, i.e. switch components generating the output voltage. The current-carrying capacity of the largest IGBT components is several hundreds of amperes and their maximum voltage is thousands of volts. In frequency converters, switch components are used purely as switches, in which case they have two states, i.e. fully conducting and fully blocking, in practice. Changing states is carried out as fast as possible to avoid a concurrent voltage and current in the component.
The IGBT mentioned herein is a component made up of several parts and, at the same time, of several pieces having different thermal resistances. The semiconductor components can be considered to be formed by a baseplate, substrate, and the actual semiconductor elements, i.e. chips. The function of the baseplate is to conduct heat generated in the component to cooling elements, such as heat sinks or the like. The substrate is on top of the baseplate and the chip is fastened thereto. It is clear that the chip, being a resistive component, heats up the fastest and the most due to the current running through the component. The baseplate, in turn, heats up the slowest and the least of the component parts, i.e. it has the highest temperature time constant in part due to cooling and in part due to the distribution of heat to the large volume of the baseplate.
The different parts of the semiconductor components have not only different temperature time constants, but also different thermal expansion coefficients. A thermal expansion coefficient indicates the size of expansion caused by a temperature change in an object. Because the parts of a semiconductor component are tightly together, often soldered, mechanical forces occur between them due to expansions of different sizes and strain the component and eventually destroy it.
This problem of heat stress is especially serious when power semiconductors are loaded in cycles. A cyclic load refers to a load that is not even, but is formed by situations in which a power semiconductor's load is high for a time and low thereafter. Such a load generates a lot of temperature variation in the power semiconductor as the temperature rises strongly during a high load, i.e. current, and drops as the load is reduced. Such a cyclic load ages a power semiconductor prematurely.
Examples of cyclic drives of an inverter include crane, centrifuge and lift drives. For instance in centrifuge drives, an inverter controls the motor to rotate the centrifuge that requires a high torque to accelerate, which means a high current and a high temperature increase in the semiconductor. After acceleration, the centrifuge is rotated at an operating speed, in which case the output current of the inverter decreases significantly as the required torque decreases. The semiconductor component that heated up during the acceleration now starts to cool. If the centrifuge is further slowed regeneratively, i.e. by using the motor as a generator, a high current again passes through the switch components and the components heat up. The same applies to lift and crane drives and other cyclic drives.
One current method of dimensioning drives is to carry it out on the basis of the semiconductor temperature variation, i.e. amplitude, caused by the cyclic load. Semiconductor manufacturers indicate a probable number of cycles endured by the semiconductor as a function of temperature variation. As the temperature variation decreases, the allowed maximum number of cycles increases.
Today, frequency converters, like other electric devices, are cooled actively by fans or liquid cooling. In such solutions, heat is transferred away from the device to cool the components of the device that heat up. Generally, the power semiconductor components are attached to a cooling element, such as heat sinks or the like. Further, the temperature is kept low by utilizing forced air or liquid in connection with the heat sink. Usually constant cooling is used, in which case, despite the temperature of the power semiconductors, a cooling fan or pump operates at a constant rotation rate. In some solutions, it is also known to use cooling that changes directly according to temperature, in which case, as the production of heat increases, cooling power is increased to restrict the maximum temperature.
It is clear that by using a fan to cool the components, the maximum temperature of the components can be brought down. Thus, the operation of the fan is taken into account when dimensioning the power semiconductors such that the highest allowable temperature of the component will not be exceeded.
The use of forced air or liquid cooling only does not, however, make it possible to decrease the temperature profile, i.e. the difference between the highest and the lowest temperatures of the component, so as to avoid problems caused by temperature changes especially in cyclic drives.